Ok, I was looking inside my old machine trying to figure out why it sounds like a freight train. There is a lot of dust in general (see where I swiped my finger...), and the CPU fan is just NASTY.

Is the best way to clean this with compressed air? Is kicking all that dust around going to hurt anything inside the machine?

On a side note, I now think the fan that is making most of the noise is this CPU fan. I can't figure out how to make it run slower, and suggestions would be appreciated. I don't see anything in the bios, I tried the Q-fan program in the Asus utility suite and nothing happened, and I don't see a setting in XP like there is in Win 7.

Atomic, I am really perplexed. I have cleaned out the dust the best I can, but read on.

This is a Q6600 with a Zalman CNPS 9500 cooler. The fan monitor in the bios reports a speed of about 2650 which is full bore. I have checked to see if the monitor is working by slowing the fan down by touching it, and it seems to work. CPU temps are at 23-24 C at 2.4 GHz or 29-30 OC'd to 2.9.

The CPU fan spins full bore like this even when the system is asleep! I have tried changing the fan settings in the bios to run at 60% but nothing changes.

Basically, if it was working properly before... it could be that the control chip for the fan itself has become borked, and is forcing on at 100%, all of the time. Which I suppose is better than shutting the fan off.

It could be that your motherboard header is no longer being controlled properly by the motherboard. You could try installing speedfan and trying to manually tune the cpu fan speed...

ps: basic fan lesson... old 3 wire fans are voltage controlled. it uses 2 wires to supply varying levels of voltage to the fan, to change the speed... the third wire is an rpm monitor. 4 wire pwm fans use pulse width modulation to control the fan speed... 2 wires supply the 12 volts for the fan, third wire is rpm monitor, 4th wire sends the pulses to the fan that control the fan speed. (the fan has smarts inside to interpret x number of pulses per millisec equals y speed, etc etc) Pretty sure in the absence of a pwm signal on the 4th wire, most pwm fans will default to 100% speed. Someone chime in here if they know for sure on this one.

I tried figuring out Speedfan earlier and it just confused me. I'm thinking of taking one of the low noise adapters (voltage limiter) from the Noctua cooler I just installed and seeing if I can use that to drop the voltage on the Zalman.

You know, one thing that is interesting is that the mobo was replaced on this machine at one point, switching from Gigabyte to Asus in the process. I wonder if that change might have caused something...

I wanted to add, just compressed air isn't normally enough in my opinion. I clean my PC every 3 months or so and I can not stand one spec of dust being left behind. I usually come at it with a bottle of alcohol, an army of Q-Tips and a thin, lint free cloth. The Q-Tips with alcohol get in between all of the nooks and crannies. Then I use the cloth and canned air afterward to do a final wipe down and get rid of the remnants of dust an Q-Tip lint. It takes some time, but it's worth it.